The invention is directed to a method for manufacturing an electrically conductive metallic strip for the production of plug contact elements.
Plug contact connections are widely used in electrical applications. Basically, these are mechanical arrangements of contact elements for the possibly repeated opening and closing of one or several electrically conducting connections. Plug contact connections find use in extremely varied applications, for instance in motor vehicle electrical systems, information technology or industrial plant electronic systems.
It is important that the contact elements of a plug connection reliably provide an electrically well conducting connection frequently over a long period of time and under the existing ambient mechanical, electrical and climatic conditions, as well as provide for the safe disconnection of the circuit in question. Depending on the area of application, there are many embodiments of the connection in question.
A typical method for the production of such plug contact elements is to punch them out of a copper or copper alloy strip. Copper has a high degree of electrical conductivity. The copper/copper alloy strips are often tin plated to provide protection against corrosion and wear as well as to increase the surface hardness of the contact element. Tin is often used as the coating on copper because of its good corrosion resistance properties. Also, if the plug contacts are insulated with rubber, the tin coating keeps the sulfur contained in the rubber away from the copper.
Metallic tin coatings are customarily produced by electroplating, hot dipping, spray metallization, cladding, diffusion or chemical vapor deposition.
The hot dipping method in the form of hot dip tinning is in widespread use for plug contact elements. For this purpose, the copper or copper alloy strip is guided through a liquid mass of molten tin. As the result of diffusion processes between the metal atoms of the liquid tin and the copper atoms, an alloy layer is formed. When the strip is removed from the bath, it is coated with a layer of tin. Excess adhering tin is removed. This takes place by mechanically scraping the strip. In addition, the surplus tin can be removed by blown off with the aid of air or protective gas.
The coating thickness is subject to a relatively low variation. However, as compared with a rolled bright strip surface, it has higher surface roughness and is uneven. The tin-plated strip has a surface hardness corresponding to that of tin or tin alloy. The characteristics of the surface are defined by the deposition and stripping process steps from the molten state. The roughness of the surface contributes to higher insertion and withdrawal forces of any plug contact element made from the starting strip. Furthermore, the uneven surfaces result in tin abrasion in the punching die.
In this connection, a known method is to roll the tinplated strip once more in order to obtain strain-hardening of the tin plating and leveling of the surface. However, the outcome obtained in this manner with the known parameters is inadequate.